If you want to clean up dead blooms trim deadheads off after blooming. I personally don't cut my mums back of their fall foliage till spring. Keeping foliage over winter helps feed the roots. So in spring I cut dead foliage before they start growing again. Once they are growing, I cut my mum back to 4inches on July 4th, so they don't fall apart when loaded with blooms. Cutting them back at this time also increase the amount of blooms.